1. Field of the Invention
The present invention relates to a metal-ceramic composite and to a vacuum switch unit using the same.
2. Description of the Related Art
As a switch for selectively switching the supply of current for controlling application of a high-voltage load, a vacuum switch unit is being widely used to prevent sparking attendant on current shutoff and subsequent discharge short to thereby secure sufficient insulation properties. The vacuum switch unit includes an internally evacuated ceramic container for enclosing a contact. In such a vacuum unit, typically, the ceramic container is formed from a cylindrical ceramic member, with its opposite ends being covered by a metallic covering member. The metallic covering member and the ceramic member are brazed to each other to form a metal-ceramic composite.
To hermetically join a metal member to the end faces of the cylindrical ceramic member, an edge seal structure or a butt seal structure can be employed. In the former, the end face of a tubular portion of the metal member is butt joined via a brazing material to the end face of the cylindrical ceramic member. In the latter, the planar surface of a plate-like portion of the metal member such as a metal lid laps over the end face of the ceramic member, with a brazing material being interposed therebetween for joining the confronting faces. In particular, conventional vacuum switch units have hitherto employed either the edge seal structure or the butt seal structure irrespective of the outer diameter or the wall thickness of the ceramic member. For example, Japanese Patent Laid-open Pub. No. Sho 52-59863 discloses making the metal member from an alloy having a low coefficient of expansion such as Kovar to thereby obtain a good composite with few defects no matter whether an edge seal or a butt seal is used. Japanese Patent Laid-open Pub. No. Hei 7-172946 describes, on the assumption that a butt seal structure is employed, that a good composite can be obtained by defining the ratio of the wall thickness of the ceramic member to the thickness of the lapped joint portion of the metal member and by defining the coefficient of linear expansion of the material or the melting point of the brazing material.
In terms of the technique disclosed in the above patent publication, however, a problem may still remain. For example, if the ceramic member has variously different outer diameters depending on the product, the joint structure employed therein may possibly cause a stress concentration at a particular site of the brazing joint. This is due to different coefficients of linear expansion between the ceramic member and the metal member, at the time of cooling after brazing, thus tending to produce defective units having an extremely low joint strength.
It is therefore the object of the present invention to provide a metal-ceramic composite having a high joint strength which is less likely to cause defects such as cuts or cracks at the joint, by employing an optimum joint structure depending on the dimensions of the ceramic member, and to provide a vacuum switch unit using such a metal-ceramic composite.
The above object has been achieved, according to a first aspect of the present invention, by providing a metal-ceramic composite comprising a metal member having a tubular portion and a cylindrical ceramic member, the metal-ceramic composite having a structure in which an end face of the tubular portion of the metal member is butt joined via a joint metal layer to an end face of the cylindrical ceramic member in an edge sealed manner, wherein the joint metal layer and the ceramic member contact each other at an annular region of W (mm) in average width which extends circumferentially about the end face of the ceramic member, and wherein W and D have respective values which satisfy:
Dxe2x89xa730xe2x80x83xe2x80x83{circle around (1)};
and
(1/60)xc3x97Dxe2x89xa6Wxe2x89xa6(D/30)+3.1xe2x80x83xe2x80x83{circle around (2)}
where D (mm) is an outer diameter of the ceramic member.
According to a second aspect, the present invention provides a vacuum switch unit comprising the metal-ceramic composite according to the present invention; a pair of covering members which cover opposite ends of the ceramic member, for forming a switch covering space within the interior of the ceramic member which is cylindrically shaped; and a pair of switch electrode members arranged so as to be able to come closer to or away from each other within the switch covering space, the pair of switch electrode members providing a switch contact portion.
The inventors extensively studied the relationship between the form of the joint structure and the dimensions of the ceramic member when the metal member is joined to the tubular ceramic member by way of a joint metal layer (e.g., brazing material). Consequently, for a ceramic member having a 30 mm or more outer diameter (the above condition {circle around (1)}) which is generally widely used for the vacuum switch unit, etc., the present inventors discovered that when the width W of the joint region becomes smaller than the outer diameter D, cuts or other defects tend to occur at the inner edge or outer edge of the contact region. Further examination led to the conclusion that such cuts easily occur when a butt seal structure is adopted of the two kinds of joint structures.
Based on these studies, experiments were carried out to identify the conditions for achieving a good joint. It was found that even though the thickness of the ceramic member cannot be made large enough against its outer diameter D, hence the joint region width W becomes less than a certain limit value, more specifically, the upper limit Wmax expressed by Wmax=D/30+3.1, adoption of the edge seal structure in particular as the joint structure would resist cuts or other defects in the ceramic member arising from stress concentration at the inner edge or the outer edge of the joint region and could ensure a composite having a high joint strength. The present invention was thus completed. Application of such a composite to the vacuum switch unit would make it possible to enhance the durability of the joint formed between the ceramic member, i.e., the switch outer envelope and the metallic covering member, to realize a vacuum switch having a long service life which hardly develops leaks or other deficiencies. Reduction of such defective joints contributes to a remarkable improvement in product yield.
When the joint region width W is less than Wmax, adoption of other joint structures, e.g., a butt seal structure as opposed to the edge seal structure, may induce a remarkable occurrence of the above defects at the joint, resulting in lower durability attributable to insufficient strength at the joint and in lower product yield arising from an increase in the defect rate. When the joint region width W is larger than Wmax, it is not so advantageous to adopt an edge seal structure, since a good joint structure can also be obtained from another method such as the butt seal method. For example, the butt seal structure is comparatively simple; the only thing needed is to lap join the lid-shaped metal member to the end face of the ceramic member by way of a brazing layer, whereas in the edge seal structure, a cylindrical portion is indispensable for the butt joint, and the vacuum switch unit must be equipped with separate metal lids to cover the internal space. This adds to the number of parts and steps, which leads to comparatively high costs. Consequently, when W is more than Wmax, it is sometimes disadvantageous in terms of cost to adopt an edge seal structure.
On the other hand, when the joint width W is less than the lower limit Wmin which satisfies Wmin=(1/60)xc3x97D (mm), the joint area against the outer diameter D of the ceramic member becomes relatively too small, and hence it becomes impossible to achieve sufficient joint strength. Furthermore, a joint width W less than Wmin may induce a lowering of the strength of the ceramic member itself because of reduced thickness.